I just finished creating some charts for my Django app using Highcharts and had to group some data for them. I thought it would be just as simple as calling a group_by() method but turned out it works a little differently but still pretty straightforward and well documented.

Just to show an example (I’m using Django 1.6):

# glucoses/models.py

class Category(models.Model):
    name = models.CharField(unique=True, max_length=255)


class Glucose(TimeStampedModel):
    user = models.ForeignKey(User)
    value = models.PositiveIntegerField() # in mg/dL
    category = models.ForeignKey('Category')
    ...

If I want to group the data by category, take the average of the values , and count the number of records for that group, I would write something like this:

from django.db.models import Avg, Count

from glucoses.models import Glucose

data = Glucose.objects.values('category__name')\
    .annotate(num_values=Count('value'), average=Avg('value'))\
    .order_by('-average')

The output is a ValuesQuerySet object that looks something like this:

[
    {'category__name': u'Bedtime', 'average': 154.62, 'num_values': 51},
    {'category__name': u'Dinner', 'average': 151.2, 'num_values': 60},
    {'category__name': u'Lunch', 'average': 144.73, 'num_values': 73},
    {'category__name': u'Breakfast', 'average': 142.17, 'num_values': 57}
]

The key here for grouping by field is the values() clause. You can then use annotate() to do calculations for the grouped values.